# Torquing a crank bolt without a torque wrench.

#

**1**Senior Member

Thread Starter

Join Date: Mar 2014

Posts: 75

Mentioned: 0 Post(s)

Tagged: 0 Thread(s)

Quoted: 0 Post(s)

Likes: 0

Liked 0 Times
in
0 Posts

**Torquing a crank bolt without a torque wrench.**

I don't have a torque wrench so I'm going to have to rely on feel or ask my LBS to do it for me.

Ok, so Torque = Radius * Force

I weigh 150 lb which is about 676 Newtons. When using the wrench, I use press down at a point about 7 inches from the center of rotation which approximates to 0.1778 meters.

For the bottom bracket, the recommended torque is 50 - 70 N·m. For the crank, the recommended torque is 35-50 N*m .

Full body Weight - 7 inches/0.1778m from axis of rotation

T=676[N]*0.1778[m]= 120.[Nm]

Full body weight is obviously too much.

Full body Weight - 3.5[in]/0.0889[m] from axis of rotation

T=676[N]*0.1778[m]/2=60.1[Nm]

Much better for the bottom bracket as it's within range. Of course, I can't put my full body weight onto that without standing on the wrench which would be absurd.

Now I just need to know how much Force I can exert on a wrench when pushing down on it.

Anyone who can help me out with this will be greatly appreciated.

Ok, so Torque = Radius * Force

I weigh 150 lb which is about 676 Newtons. When using the wrench, I use press down at a point about 7 inches from the center of rotation which approximates to 0.1778 meters.

For the bottom bracket, the recommended torque is 50 - 70 N·m. For the crank, the recommended torque is 35-50 N*m .

Full body Weight - 7 inches/0.1778m from axis of rotation

T=676[N]*0.1778[m]= 120.[Nm]

Full body weight is obviously too much.

Full body Weight - 3.5[in]/0.0889[m] from axis of rotation

T=676[N]*0.1778[m]/2=60.1[Nm]

Much better for the bottom bracket as it's within range. Of course, I can't put my full body weight onto that without standing on the wrench which would be absurd.

Now I just need to know how much Force I can exert on a wrench when pushing down on it.

Anyone who can help me out with this will be greatly appreciated.

#

**2**Hogosha Sekai

Join Date: Jun 2011

Location: STS

Posts: 6,670

Bikes:

*Leader 725, Centurion Turbo, Scwhinn Peloton, Schwinn Premis, GT Tequesta, Bridgestone CB-2,72' Centurion Lemans, 72 Raleigh Competition*Mentioned: 17 Post(s)

Tagged: 0 Thread(s)

Quoted: 70 Post(s)

Likes: 0

Liked 14 Times
in
13 Posts

Get a torque wrench and a hex set for it.. you'll be glad you did. Or go to the LBS and have them do it, guestimating nm's and whatnot isn't going to work very well.. especially since we can't definitively say how much force you put out with your arms and that wrench.

#

**3**Senior Member

Thread Starter

Join Date: Mar 2014

Posts: 75

Mentioned: 0 Post(s)

Tagged: 0 Thread(s)

Quoted: 0 Post(s)

Likes: 0

Liked 0 Times
in
0 Posts

Ok, I suppose I'll take it down to my LBS then.

And here I was, thinking I could put my college physics skills to use...

And here I was, thinking I could put my college physics skills to use...

#

**4**Keepin it Wheel

Join Date: Aug 2011

Location: San Diego

Posts: 9,484

Bikes:

*Surly CrossCheck, Krampus*Mentioned: 0 Post(s)

Tagged: 0 Thread(s)

Quoted: 20 Post(s)

Liked 1,372 Times
in
1,018 Posts

I like your approach, and would like to hear responses from other DIY types who can advise. Sorry I can't, but I like your idea of choosing a point along the lever arm that gives the appropriate torque from your bodyweight. Maybe localize the pressure by putting a rope around the handle at the desired radius and leaning on the rope?

#

**5**Senior Member

Join Date: Dec 2010

Location: Above ground, Walnut Creek, Ca

Posts: 6,681

Bikes:

*8 ss bikes, 1 5-speed touring bike*Mentioned: 0 Post(s)

Tagged: 0 Thread(s)

Quoted: 86 Post(s)

Likes: 0

Liked 2 Times
in
2 Posts

by all means try it, hate to see a good physics education going to waste. OTOH, i use the 1 grunt, 2 grunt, or occasionally the 3 grunt metric...

#

**6**Hogosha Sekai

Join Date: Jun 2011

Location: STS

Posts: 6,670

Bikes:

*Leader 725, Centurion Turbo, Scwhinn Peloton, Schwinn Premis, GT Tequesta, Bridgestone CB-2,72' Centurion Lemans, 72 Raleigh Competition*Mentioned: 17 Post(s)

Tagged: 0 Thread(s)

Quoted: 70 Post(s)

Likes: 0

Liked 14 Times
in
13 Posts

Sidenote: If you're in wine country you can bring your bike on over and we can can figure out your output using my Torque wrench

#

**7**Mechanic/Tourist

Join Date: Apr 2007

Location: Syracuse, NY

Posts: 7,510

Bikes:

*2008 Novara Randonee - love it. Previous bikes:Motobecane Mirage, 1972 Moto Grand Jubilee (my fave), Jackson Rake 16, 1983 C'dale ST500.*Mentioned: 10 Post(s)

Tagged: 0 Thread(s)

Quoted: 482 Post(s)

Likes: 0

Liked 6 Times
in
6 Posts

OK, this is silly. Going to a shop is fine. For those who can't do that just use a 7-8 inch wrench and put as much force as you reasonably can on it by hand (not standing on it - close enough.

If you are concerned about getting at least roughly the right amount of torque here's a diy solution off the top of my head. Do the calculation for what lbs/kgs of pressure is needed for the length of wrench you have (using the center of your hand on the wrench as the distance). Push down on a bathroom scale a few times until you have a sense of how that pressure feels. Loosen the bolt then tighten to that same feel.

If you are concerned about getting at least roughly the right amount of torque here's a diy solution off the top of my head. Do the calculation for what lbs/kgs of pressure is needed for the length of wrench you have (using the center of your hand on the wrench as the distance). Push down on a bathroom scale a few times until you have a sense of how that pressure feels. Loosen the bolt then tighten to that same feel.

__________________

There's no such thing as a routine repair.

Don't tell me what "should" be - either it is, it isn't, or do something about it.

If you think I'm being blunt take it as a compliment - if I thought you were too weak to handle the truth or a strong opinion I would not bother.

There's no such thing as a routine repair.

Don't tell me what "should" be - either it is, it isn't, or do something about it.

If you think I'm being blunt take it as a compliment - if I thought you were too weak to handle the truth or a strong opinion I would not bother.

*Please respect others by taking the time to post clearly so we can answer quickly. All lowercase and multiple typos makes for a hard read. Thanks!*
#

**8**Senior Member

Join Date: Jan 2013

Location: Llano Estacado

Posts: 3,702

Bikes:

*old clunker*Mentioned: 13 Post(s)

Tagged: 0 Thread(s)

Quoted: 684 Post(s)

Likes: 0

Liked 102 Times
in
80 Posts

1. The force must be tangent to the radius (perpendicular to the wrench).

2. Torque values for threaded fasteners assume dynamic torque. The fastener must be turning at the moment the desired value is reached.

#

**9**Senior Member

Thread Starter

Join Date: Mar 2014

Posts: 75

Mentioned: 0 Post(s)

Tagged: 0 Thread(s)

Quoted: 0 Post(s)

Likes: 0

Liked 0 Times
in
0 Posts

I shall begin planning for my cross country expedition from the East coast to visit your humble abode so that I may partake in your wisdom and borrow your torque wrench.

#

**10**Senior Member

Thread Starter

Join Date: Mar 2014

Posts: 75

Mentioned: 0 Post(s)

Tagged: 0 Thread(s)

Quoted: 0 Post(s)

Likes: 0

Liked 0 Times
in
0 Posts

If you are concerned about getting at least roughly the right amount of torque here's a diy solution off the top of my head. Do the calculation for what lbs/kgs of pressure is needed for the length of wrench you have (using the center of your hand on the wrench as the distance). Push down on a bathroom scale a few times until you have a sense of how that pressure feels. Loosen the bolt then tighten to that same feel.

#

**12**Senior Member

Join Date: Mar 2012

Location: southeastern PA - a mile west of Philadelphia

Posts: 430

Mentioned: 0 Post(s)

Tagged: 0 Thread(s)

Quoted: 0 Post(s)

Likes: 0

Liked 0 Times
in
0 Posts

Admittedly, the use of a simple torque wrench is the easiest method to employ here however, in the event a torque wrench is unavailable, the following method allows the required leverage point on a lever arm to be determined, at which point an individual’s full body weight can be applied to the lever arm (while the lever arm is perpendicular to gravity’s vertical influence) as measured from the crank arm’s axis of rotation to produce the desired torque.

Suppose we don’t have access to a torque wrench, but we’re required to apply 50 Nm (Newton meters) of torque to tighten the crank arm bolts to their max 50 Nm torque rating. We’ll need to determine the precise point on a lever arm at which to apply our full body weight of 150 pounds to produce the required torque. What distance from the crank arm’s axis of rotation is required to accomplish this task?

Start by converting your body weight “in pounds” to a body weight as measured “in Newtons” and this is easily accomplished as follows:

Newtons = pounds / .224961492

Hence,

150 pounds / .224961492 =

Next, we apply the desired 50 Nm of torque and our 666.780784 Newtons of body weight as follows:

50 Nm / 666.780784 Newtons =

We’ve now determined the precise distance (.074987164 meters) away from the crank arm’s axis of rotation that our 666.780784 Newton body weight must be applied on the lever arm in order to produce the desired 50 Nm of torque.

If you need to convert the derived lever arm distance from meters to inches, simply divide the distance yielded “in meters” by the mathematical constant of .0254 as shown below:

.074987164 meters / .0254 = 2.952250551 inches (which is just several 64[SUP]ths[/SUP] of an inch short of 3 inches)

And so, the point on the lever arm at which we must apply our full body weight of 666.780784 Newtons is just a tad less than

We can verify the accuracy of our derived leverage point by merely applying it via the following long established physics equation:

T = F * r

Whereby,

Torque “in Nm”= Force “in Newtons” * radius “in meters”

Hence,

666.780784 Newtons * .074987164 meters = 50 Nm of torque

As demonstrated, the precise lever arm length was derived by these straightforward calculations.

For those interested in the precise conversion factor to convert Nm of torque into foot pounds of torque:

Nm of torque * .738062638 = foot pounds of torque

Though it isn’t necessary to use 10 digits worth of calculator precision, I prefer to do so in order to acquire precision outcomes that can afterward be rounded to however many digits of precision is deemed satisfactory.

**Scenario:**Suppose we don’t have access to a torque wrench, but we’re required to apply 50 Nm (Newton meters) of torque to tighten the crank arm bolts to their max 50 Nm torque rating. We’ll need to determine the precise point on a lever arm at which to apply our full body weight of 150 pounds to produce the required torque. What distance from the crank arm’s axis of rotation is required to accomplish this task?

Solution:Solution:

Start by converting your body weight “in pounds” to a body weight as measured “in Newtons” and this is easily accomplished as follows:

Newtons = pounds / .224961492

Hence,

150 pounds / .224961492 =

**666.780784 Newtons**of body weightNext, we apply the desired 50 Nm of torque and our 666.780784 Newtons of body weight as follows:

50 Nm / 666.780784 Newtons =

**.074987164 meters**We’ve now determined the precise distance (.074987164 meters) away from the crank arm’s axis of rotation that our 666.780784 Newton body weight must be applied on the lever arm in order to produce the desired 50 Nm of torque.

If you need to convert the derived lever arm distance from meters to inches, simply divide the distance yielded “in meters” by the mathematical constant of .0254 as shown below:

.074987164 meters / .0254 = 2.952250551 inches (which is just several 64[SUP]ths[/SUP] of an inch short of 3 inches)

And so, the point on the lever arm at which we must apply our full body weight of 666.780784 Newtons is just a tad less than

**2 and 61/64 inches**away from the crank arm’s axis of rotation.We can verify the accuracy of our derived leverage point by merely applying it via the following long established physics equation:

T = F * r

Whereby,

Torque “in Nm”= Force “in Newtons” * radius “in meters”

Hence,

666.780784 Newtons * .074987164 meters = 50 Nm of torque

As demonstrated, the precise lever arm length was derived by these straightforward calculations.

For those interested in the precise conversion factor to convert Nm of torque into foot pounds of torque:

Nm of torque * .738062638 = foot pounds of torque

*** .738062638 =**

50 Nm50 Nm

**36.90313189 foot pounds**of torqueThough it isn’t necessary to use 10 digits worth of calculator precision, I prefer to do so in order to acquire precision outcomes that can afterward be rounded to however many digits of precision is deemed satisfactory.

#

**13**Senior Member

Join Date: Oct 2013

Location: Roanoke, Va

Posts: 997

Bikes:

*2013 leader 721. 2015 leader 725. 2012 fuji feather*Mentioned: 0 Post(s)

Tagged: 0 Thread(s)

Quoted: 0 Post(s)

Likes: 0

Liked 0 Times
in
0 Posts

#

**14**Senior Member

Join Date: Jan 2013

Location: Llano Estacado

Posts: 3,702

Bikes:

*old clunker*Mentioned: 13 Post(s)

Tagged: 0 Thread(s)

Quoted: 684 Post(s)

Likes: 0

Liked 102 Times
in
80 Posts

Too bad significance does not work that way. Arithmetic can be dangerous in the wrong hands. FYI, force is a vector.

#

**15**Banned

After a few decades of using a square taper crank .. and Shop work ...

A crank bolt wrench will work , just that over the web forum, ' how tight' is more easily transmitted

by using a standard unit of measure , and to have that the scale on a torque wrench is useful .

A crank bolt wrench will work , just that over the web forum, ' how tight' is more easily transmitted

by using a standard unit of measure , and to have that the scale on a torque wrench is useful .

#

**16**Senior Member

Join Date: Jul 2008

Location: Seattle

Posts: 246

Bikes:

*91 Trek franken '81 Schwinn Voyager*Mentioned: 1 Post(s)

Tagged: 0 Thread(s)

Quoted: 9 Post(s)

Likes: 0

Liked 0 Times
in
0 Posts

The nut on the end of the axel on my old VW van required 300' lbs of torque. My 1/2" wrench didn't register that high but at about 150lbs, I snugged it up by standing on the end of a 2' breaker bar. It was probably not accurate to several decimal points but I drove it another severeal years...

#

**17**Banned

over tightening the crank bolt in square taper cranks stretches the aluminum and ruins the crank arm.

#

**18**Cottered Crank

Join Date: Aug 2010

Location: Chicago

Posts: 3,493

Bikes:

*1954 Raleigh Sports 1974 Raleigh Competition 1969 Raleigh Twenty 1964 Raleigh LTD-3*Mentioned: 0 Post(s)

Tagged: 0 Thread(s)

Quoted: 6 Post(s)

Likes: 0

Liked 3 Times
in
2 Posts

Harbor Freight $10 on sale right now.

Good to well under the 4% it is guaranteed to be. I've checked mine often and it is always under 2% error.

If you really can't trust a $10-20 tool then spend another $30 and check the two against each other. It's super-easy. As long as the two agree then you are

Good to well under the 4% it is guaranteed to be. I've checked mine often and it is always under 2% error.

If you really can't trust a $10-20 tool then spend another $30 and check the two against each other. It's super-easy. As long as the two agree then you are

*Good to Go.*
#

**19**Senior Member

Thread Starter

Join Date: Mar 2014

Posts: 75

Mentioned: 0 Post(s)

Tagged: 0 Thread(s)

Quoted: 0 Post(s)

Likes: 0

Liked 0 Times
in
0 Posts

My bottom bracket and crank use Octalink V2 so it has splines. Based on the design of Octalink, I wouldn't expect ovrtorquing to have as serious a consequence. I still don't want to take any chances on that though.

#

**20**Calamari Marionette Ph.D

If you are concerned about getting at least roughly the right amount of torque here's a diy solution off the top of my head. Do the calculation for what lbs/kgs of pressure is needed for the length of wrench you have (using the center of your hand on the wrench as the distance). Push down on a bathroom scale a few times until you have a sense of how that pressure feels. Loosen the bolt then tighten to that same feel.

#

**21**Banned

My bottom bracket and crank use Octalink V2

did not advance your desire for specific answers much .

back to my previous statement .

by using a standard unit of measure , and to have that the scale on a torque wrench is useful .

40 years as a bike mechanic I have the feel for sufficient ,

but my hands can only type words and numbers over the Net.

Buy the tools , hire the work done , or just obsess over something else.

*Last edited by fietsbob; 03-06-14 at 02:12 PM.*

#

**22**Mechanic/Tourist

Join Date: Apr 2007

Location: Syracuse, NY

Posts: 7,510

Bikes:

*2008 Novara Randonee - love it. Previous bikes:Motobecane Mirage, 1972 Moto Grand Jubilee (my fave), Jackson Rake 16, 1983 C'dale ST500.*Mentioned: 10 Post(s)

Tagged: 0 Thread(s)

Quoted: 482 Post(s)

Likes: 0

Liked 6 Times
in
6 Posts

Problem is determining lever length, and I would think it would be easiest to push down on the wrench, which means you would check for the amount of decrease in the weight. .

__________________

There's no such thing as a routine repair.

Don't tell me what "should" be - either it is, it isn't, or do something about it.

If you think I'm being blunt take it as a compliment - if I thought you were too weak to handle the truth or a strong opinion I would not bother.

There's no such thing as a routine repair.

Don't tell me what "should" be - either it is, it isn't, or do something about it.

If you think I'm being blunt take it as a compliment - if I thought you were too weak to handle the truth or a strong opinion I would not bother.

*Please respect others by taking the time to post clearly so we can answer quickly. All lowercase and multiple typos makes for a hard read. Thanks!**Last edited by cny-bikeman; 03-06-14 at 03:41 PM.*

#

**23**Senior Member

Join Date: Nov 2010

Location: St. Louis, MO

Posts: 209

Bikes:

*2013 Trek Madone 4.5, 2013 Trek Domane 4.3, 2009 Trek FX 7.6*Mentioned: 0 Post(s)

Tagged: 0 Thread(s)

Quoted: 4 Post(s)

Liked 0 Times
in
0 Posts

DIY doesn't mean doing it without the proper tools. Nashbar sells a nice torque wrench kit. Should last a lifetime.

#

**25**Banned

Sort of the opposite.

so the % of your weight on the scale would be less..

then the length of the wrench handle in feet or inches or Meters

and your weight on the end of that handle in pounds, Kg.

deduced by how much less is on the Loo Scale ..

*Last edited by fietsbob; 03-06-14 at 02:57 PM.*